1. The Field of the Invention
The present invention relates to a method and system of positioning a movable body in a magnetic bearing system. More specifically, the present invention relates to a method and system of adjusting a sensor output signal, used to measure the position of the body, to account for sensor offset error due to aging, and using the adjusted signal to position the body during regular operation of the system, or to provide a starting position for the body when the system reboots or restarts.
2. The Relevant Technology
Unstable systems such as magnetic bearing systems which support a movable body without a mechanical coupling or bearing are ideally configured for fluid pumps that handle sensitive or corrosive fluids such as blood. Blood for example, can damage mechanical couplings, seals, and other pump parts with which it comes into contact. The mechanical couplings and interacting pump parts, conversely, can damage blood cells. Other fluids need to be free from contamination, mechanical shear, and other problems stemming from fluid interaction with the pump. Many food industry items require special handling to maintain their purity. Still other fluids do not respond well to increases in heat which often accompany many mechanical couplings. Thus, pumps with magnetically suspended rotors have been developed. The rotor may be made of non corrosive material for specific contact with the particular fluid being pumped. The rotor is magnetically suspended in the fluid and coupled to an isolated motor. Because the rotor is levitated actively or passively by a series of permanent or electromagnets, there is no direct mechanical interaction of pump parts to each other. The only moving part that the fluid interacts with is the rotor itself. This configuration minimizes many of the problems associated with fluid interaction with the mechanical parts of the pump.
Magnetic bearing systems, are often unstable however. Forces due to gravitation, fluid flow, vacuums, and the magnets themselves affect the position of the rotor during pumping operation. In a magnetic bearing pump, for example, if a rotor is too far out of position, not only could it adversely affect the operation of the pump, but the rotor could be overcome by one or more of the magnetic fields supporting it, causing the rotor to clamp down against a magnet preventing the pump from operating at all.
Accordingly, some magnetic bearing systems have means to detect the position of the movable body being magnetically suspended and provide a self adjusting mechanism for repositioning the movable body. One such system is the subject of U.S. Pat. No. 3,860,300, which discloses a permanent and electromagnet system. The electromagnets are used for control purposes. A persisting net force produces electrical energization of the electromagnet system resulting from displacement of the movable body. The net force is augmented until the movable body is repositioned to an oppositely displaced position of substantial equilibrium. Another self-adjusting system is disclosed in U.S. Pat. No. 5,783,8855 which discloses a self-adjusting magnetic bearing system which automatically adjusts the parameters of an axially unstable magnetic bearing such that its force balance is maintained near the point of unstable equilibrium.
The problem with many known self-adjusting magnetic bearing systems, however, is that they are sensor-based and do not account for aging or damaged sensors that may drift over time. Electrical sensors, for example, may not take the same reading for the same position of a suspended movable body as they may have when they were newer. Thus, the system may respond and react to adjust a movable position that the system determines is at a certain point, relative to a point of equilibrium, when in fact, the rotor is slightly offset from that particular portion. This phenomenon is often referred to as “sensor drift.” Sensor drift may be caused by aging, temperature variations, jarring, misalignment or other factors.
Known self-adjusting systems use output from sensors to reposition the suspended movable body. If the output is incorrect by some amount of sensor offset, a position controller may have to work too hard to reposition the movable body. This may result in damaging excess heat created by the over-worked positioner or controller. Further, it may require excess energy to reposition the movable body because of the sensor offset error caused by sensor drift. In mechanical bearing systems such as heart pumps, which are quite small and house very small batteries, excessive energy drains can significantly reduce the life of the pump.
Perhaps the most severe disadvantage of known self-adjusting magnetic bearing systems is that they do not account for sensor offsets or errors, due to sensor drift or other factors, in the event that they have to reboot or restart. Reboots or restarts may be necessary when the battery loses its power or a computer malfunction occurs. Some magnetic bearing systems come with factory sensor output settings, but sensors drift over time and the factory sensors may not be adequate for reboots or restarts after a period of time. Some positioners or controllers include certain limits to improve or account for transient response. If the sensor offset is not accounted for in the sensor reading output, it is possible that the controller cannot compensate for the offset upon reboot or restart. The result would be a large power dissipation or even failure of the movable body or rotor to levitate. This problem could be fatal in magnetic bearing systems such as heart pumps, residing in people with weak hearts.
Thus, it would be an advancement in the art to provide a magnetic bearing system and method for positioning a movable body that accounts for sensor offset or error, due to sensor drift or other factors. It would be an additional advancement in the art to provide such a system and method that could adjust a sensor output to account for sensor offset to ease the burden on magnetic bearing system positioner or position controller. It would be an additional advancement in the art to provide such a system and method that could store an estimated sensor offset adjustment and use the stored estimate or adjustment to position the movable body in the event of a restart or reboot. Such a system and method in accordance with the present invention is disclosed and claimed herein.